1. Technical Field
This invention relates generally to fixed cutter drill bits of the type used in cutting through rock formation such as when drilling for oil or the like. More particularly, this invention is concerned with the arrangement of the cutter elements which are mounted on the face of the drill bit.
2. Background Information
Typically, fixed cutter bits include cutting elements protruding from the face of a drill bit body and cutting edges of the cutter elements are arranged to define a cutting profile extending generally radially outward from the center of the bit. The cutting elements are placed at selected radial positions with respect to the central axis of the bit so that each element cuts a groove or kerf in the formation as the bit is rotated. In a usual arrangement, the profile of the cutting elements is such that the cutting edges of adjacent elements overlap each other on progressing radially outward from the central axis of the bit. As the cutting edges of the individual elements are worn during drilling, they tend to assume a profile presenting a relatively flat and single continuous cutting edge from one element to the next. This wear, of course, effects the penetration rate at which the bit effectively drills.
One example of a prior art bit is shown in U.S. Pat. No. 5,033,560 which discloses a fixed cutter bit having radial wings with different sizes of cutting elements in each of the wings. However, at any one radial position, all of the cutting elements are of the same size. In U.S. Pat. No. 4,602,691 (RE 33,757), there is disclosed a fixed cutter bit utilizing cutting elements of different sizes and shapes. Specifically, sharp elements at spaced radial positions cut small relief kerfs in the formation. Thereafter, round or blunt cutting elements follow at an adjacent radial position and dislodge the formation between the kerfs.
U.S. Pat. No. 4,913,244 discloses different sizes of cutters at different radial positions. Large diameter cutters are located between the central axis of the bit and gage. Smaller diameter gage cutters are located at the periphery of the bit body.